The present invention relates to a metal oxide film resistor having a tin oxide based metal oxide film coated on the surface of an electrically insulating substrate.
Conventional metal oxide film resistors are fabricated by the following procedures: a tin oxide based metal oxide film is formed on the surface of a typically rod-shaped ceramic substrate 1.5-2 mm in diameter and 5-6 mm in length); a metallic terminal cap is fitted over each end of the coated substrate to provide a connecting terminal; a wire lead is attached to each terminal cap; and the entire assembly except the leads is encapsulated by an electrically insulating and moisture-proof protective sheath. In order to reduce the temperature coefficient of resistance, antimony oxide is usually added to the tin oxide based metal oxide film.
In forming a metal oxide film on the surface of an insulating substrate in the manufacture of conventional metal oxide film resistors, a "spray method" is commonly employed. In the "spray method", a feed solution having stannic chloride (SnCl.sub.4) and a small amount of antimony trichloride (SbCl.sub.3) dissolved in a mixed solvent of water, HCl, alcohol, etc. is prepared and rods of a mullite-corundum ceramic substrate are supplied into a film depositing apparatus, the essential part of which is shown in FIG. 3, and a metal oxide film is deposited on the surfaces of the substrate rods to make metal oxide film resistors.
The apparatus shown in FIG. 3 comprises a furnace 6 that has a heat-resistant drum 7 mounted rotatably around a shaft and which has a heating element fitted in the furnace wall to ensure uniform heating of the drum. Outside the furnace are provided a feed solution supply unit 8 and an air compressing unit 9. The feed solution supply unit 8 and the air compressing unit 9 are connected to the drum 7 via pipes 11 and 12, respectively. The pipes 11 and 12 end with a nozzle 10 through which the feed solution is sprayed towards the drum.
Film deposition with the apparatus shown in FIG. 3 will proceed as follows: the feed solution is charged into the unit 8 and the mullite-corundum ceramic rods a are charged into the rotating drum 7; as the temperature in the furnace is elevated to 500.degree.-800.degree. C., the feed solution carried with compressed air is sprayed through the nozzle 10 to be deposited on the surfaces of the ceramic rods; thereafter, the spraying and heating operations are turned off and the substrate rods are taken out of the furnace. The recovered rods are transferred into a separate furnace where they are given a heat treatment at 200.degree.-300.degree. C. for a period ranging from several tens of minutes to several hours to form a thermally and electrically stable metal oxide film.
Subsequently, a metallic cap is fitted over each end of the substrate and a helix is cut through the film into the substrate to obtain a desired value of resistance. A wire lead is then welded to each cap and a protective coating is applied to make a final product of metal oxide film resistor.
The prior art metal oxide film resistors fabricated by the process described above have had the disadvantage that because of problems such as low stability and reliability of coated films, the values of resistance that can be attained before cutting the helix are only up to about 200 ohms. In the absence of helical cuts into the film on a substrate having dimensions comparable to those employed previously, higher values of resistance could be attained by decreasing the thickness of a metal oxide film to be formed on the substrate. However, this approach suffers from the disadvantage of variations in other characteristics of the resistor such as the increase in the temperature coefficient of resistance and the decrease in thermal stability, which lead to increases in the amount of change in resistance as a result of soldering of wire leads or exposure to high temperatures. Because of these limitations, the method of increasing the value of resistance by reducing the thickness of a metal oxide film has been unable to produce commercially acceptable high-resistance metal oxide film resistors.